The goal of the proposed research is to elucidate the molecule basis of chloramphenicol-induced aplastic anemia. The antibiotic chloramphenicol (CAP) binds reversibly to mitochondrial ribosomes, directly inhibiting mitochondrial protein synthesis. Prolonged exposure to CAP leads to reduced proliferative capacity of the affected cells, and this appears to be responsible for the reversible bone marrow depression during periods of CAP therapy. There appears to be a genetic predisposition for some individuals to develop aplastic anemia after CAP treatment. This usually fatal side effect of CAP therapy is of unknown etiology. We propose that CAP-induced aplastic anemia results from the irreversible inactivation of mitochondrial ribosomes in the erythropoietic stem cells of susceptible individuals. We hypothesize that the mitochondrial ribosomes of susceptible individuals contain a mutation which renders them chemically reactive towards CAP. Chemically reactive analogues of CAP will be incubated with rat bone marrow cells in culture to test this hypothesis in a model system. In addition, we will analyze the proteins (two dimensional electrophoresis) and RNA of human mitochondrial ribosomes from normal (non-susceptible) cells and cells derived from individuals who are susceptible to chloramphenicol-induced aplastic anemia. After identifying the chloramphenicol-binding protein in human mitochondrial ribosomes, we will assess its reactivity with CAP in normal and susceptible cells. Finally, this protein (or RNA) in susceptible ribosomes will be analyzed to identify the reactive substituent, thereby establishing the molecular defect, and elucidating the etiology of CAP-induced aplastic anemia.